Diego Rivera, well known for his reintroduction of large frescoes into modern art, was commissioned to create a series of illustrations for a book that would have been the first English translation of the Popol Vuh, the Maya creation story. Although the book was never published, the Library has three of the original illustrations, Creation, Trials of the Hero Twins, and Human Sacrifice before Tohil. In 2019, these items visited the Preservation Research and Testing Division (PRTD) for analytical studies. These studies help us better understand the way materials will respond to handling and display, but sometimes shed light on the history or interpretation of items. For these illustrations, it was a chance to understand how this artist worked in a medium outside his typical body of work.
Analysis began with multispectral imaging. Spectral imaging is an imaging technique that captures information within and outside the visible spectrum of light, one of its purposes being to reveal content not visible to the naked eye. In essence, this involves illuminating the object in different wavelengths of light, including a range of ultraviolet, visible, and infrared light, and capturing a datacube of images. Different materials will respond differently to each wavelength during the imaging process. The combination of all these images and the material’s various spectral responses provides a comprehension understanding of a collection item that helps inform additional complementary analyses.
Under infrared illumination, the under drawing of the watercolor is revealed because infrared light passes through the upper layers of paint until being absorbed by certain materials, such as the carbon in a pencil. Much of the original sketch becomes visible and you can see how Rivera planned out the imagery before he began painting. Though the final illustration is not perfectly aligned with the sketched under drawing, most areas do not differ significantly from the original intended design. The only exception is the sketch underneath the small snake in the Creation painting. Though hard to completely discern, it looks as if the originally intended animal was a winged animal, and Rivera changed his mind to paint a serpent there instead. There are no other significant differences in content within the other watercolors, though in the fourth panel of Trials of the Hero Twins, one of the brother’s hands slightly shifted position.
Principal component analysis (PCA) assigns arbitrary colors to areas of matching chemical composition, instead of the colors seen by eye. This is a processing method that reduces the entire image cube from every wavelength into a single image that discern similarities and differences in the spectral response of the materials used across the entire painting at once. Some pigments may look similar to the naked eye but are in fact different. In Creation for example, PCA helped confirm all the yellow pigment was the same but indicated subtle differences between some of the blues and greens. This allows us to ensure we take a closer look at these different areas with other analysis techniques that we may have otherwise assumed were the same and overlooked.
Building on top of the knowledge of pigment distributions from multispectral imaging, fiber optic reflectance spectroscopy and x-ray fluorescence spectroscopy were used to study a few areas of yellow and red and each area of greens and blues. The Creation revealed a modern palette rich in vibrant cadmium and vermilion based pigments, whereas the other paintings had a more subdued palette using earth-based pigments and few modern ones. In talking with John Hessler, curator of the Jay I. Kislak collection, this change in palette choices seems to correlate with Rivera’s move from California (where he painted the Creation) to his home studio in Mexico (where he painted the other two.) The continued discussions between scientists with curators and conservators always adds another layer of meaning to create a full story of the live of an object.
A bit surprising, is that the greenish and blueish hues on the Creation are most likely composed of Paris green, which was a risky choice. Besides being a highly toxic arsenic and copper containing pigment (used mainly as an insecticide during Rivera’s time, although still highly popular among artists), Paris green has many known degradation issues. For example, in acidic conditions (as may well be the case on his watercolor paper), Paris green will degrade into blue-green copper acetate salts and white triarsenic oxide. Even stranger, Rivera mixed cadmium yellow with Paris green to achieve various green hues. Cadmium yellow, a sulfur containing pigment, and Paris green, a copper containing pigment are, by definition, incompatible (you can read about it in Rutherford J. Gettens book, Painting Materials: A Short Encyclopedia).
The combination of these two pigments have frustrated artists and conservators for years because the two form a black copper sulfide material that has led to the darkening of many paintings. Yet, Diego Rivera seems to have mixed the two pigments together in the trees and sea animals and serpent feathers, most likely knowing the risk of darkening. Except there was no darkening. Rivera was, after all, well known for his experimentation with paints and pigments, particularly during his friendship with Picasso. Although it is currently unknown how Rivera sidestepped the cadmium yellow/Paris green degradation, it is possible that through some experimentation with Paris green and mixtures, he managed to find the exact colors he wanted, as well as a way to buffer any degradation products. The other two watercolors, Trials of the Hero Twins and Human Sacrifice before Tohil, did not contain any cadmium related or Paris green pigments and were a bit more muted in overall tone. In these watercolors, Rivera seems to have moved onto chrome-based pigments, but otherwise presented nothing unusual.
Analysis of the artist’s materials of Rivera’s Popol Vuh watercolors provide a deeper understanding of the collection items when coupled with historic insight. Understanding the chemical nature of these materials not only helps with preserving them for longevity but ensures proper handling protocol. The unique findings of this study assists scholars and curators to better understand their collections and share these insight into the Library’s collections.